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Definition

noun

A metabolic pathway where CO₂ is first added to phosphoenolpyruvate by the enzyme, PEP carboxylase, producing the four-carbon compound within mesophyll cells that is later transported to the bundle sheath cells where the CO₂ is to be released for use in the Calvin cycle.

Supplement

Instead of the direct carbon fixation in the Calvin cycle like in C₃ carbon fixation, the C₄ pathway involves steps that first converts pyruvate to phosphoenolpyruvate (PEP) to bind with the CO₂ forming a four-carbon compound (hence the name C₄). As a result, the photorespiration pathway is bypassed, and the wasteful loss of CO₂ common in C₃ carbon fixation pathway is minimized.

Plants that first go through the C₄ pathway are better adapted than plants that solely go through the C₃ pathway under conditions of drought, high temperatures and low nitrogen or CO₂ concentrations.

Word origin: from the intermediate organic compound, which contains four carbon atoms, hence the name C₄.

Synonym: hatch slack kortshak pathway, Hatch-Slack pathway.
Compare: C3 carbon fixation pathway, Crassulacean acid metabolism.
See also: C4 plant.